1. Field of the Invention
Aspects of the invention relate to an image forming apparatus, and, more specifically, to an image forming apparatus in which images are transferred and superposed onto a transfer medium by a plurality of image bearing bodies.
2. Description of the Related Art
In general, color image forming apparatuses are classified into two types: a multi-pass system which forms a color image by rotating an image bearing body (e.g., photosensitive drum) multiple times, and a single-pass (tandem type) system which forms a color image by rotating each of a plurality of image bearing bodies one time.
The multi-pass system image forming apparatus forms a color image by rotating one image bearing body multiple times, so the printing speed is relatively slower than in the single-pass system image forming apparatus. However, since the multi-pass system typically uses only a single image bearing body, a relatively small number of components are used, resulting in a simple constitution. In this sense, the multi-pass system is advantageous in that it contributes to a compact-size image forming apparatus. For instance, in the multi-pass system image forming apparatus, a plurality of color images, i.e., yellow, magenta, cyan, and black color images, which are formed through a predetermined operation, are transferred onto a transfer medium, such as a transfer belt and superposed (primary transfer) with each other. Then, the full color images superposed on the transfer medium are transferred onto a recording medium (secondary transfer). Therefore, since one image bearing body typically is rotated four turns to develop a single full-color image, the printing speed can be slow.
On the other hand, the single-pass system image forming apparatus includes four image bearing bodies corresponding to yellow, magenta, cyan and black color images. During one rotation of a transfer medium, the four color images formed on their corresponding image bearing bodies are transferred in sequence onto the transfer medium and superposed with each other to make a full color image. Thus, the printing speed is typically shortened. However, since the single-pass system generally requires an image bearing body for each color, it can be difficult to manufacture a small-sized and light-weight image forming apparatus of a low-price for consumers.
Also, an image forming apparatus having a combined system of the single-pass and the multi-pass system has been developed. According to this system, the number of image bearing bodies is less than in the single-pass system, whereas its printing speed is faster than in the multi-pass system. This image forming apparatus includes only two image bearing bodies, in which one (first) of the image bearing bodies is used to form yellow, magenta, and cyan color images based on the multi-pass system and the other (second) image bearing body is used to form a black color image. The yellow and magenta color images are superposed on a transfer medium through the first image bearing body, and then a cyan color image is finally superposed thereon. This image transfer onto a transfer medium is carried out using the single-pass system so that a full-color image can be formed on the transfer medium. In other words, to form a full-color image on the transfer medium, the first image bearing body makes first two rotations and then the first and second image bearing bodies together make a third rotation. Thus, as previously mentioned, the image forming apparatus having the combined system typically has less components and less manufacturing cost than the image forming apparatus with four image bearing bodies, and can realize a faster printing speed than in the image forming apparatus with one image bearing body.
However, in the above described image forming apparatus system, the second image bearing body is typically disengaged from the transfer medium farther than the application range until the magenta color image is formed through the first image bearing body, and is within the application range of the transfer medium when the cyan color image is formed. One way to accomplish this is to adjust the space between the first image bearing body and the transfer medium by adjusting the traveling trajectory of the transfer medium. However, such adjustment can cause the transfer medium to slip from the track and, therefore, the images on the first and second image bearing bodies can be tilted over and banding is more likely to be noticed.